A collision between two large trucks traveling in opposite directions killed Robert Eugene Conway, who was driving a truck owned by Dixie Transport of Texas, Inc. (Dixie Transport). The other truck, which was owned by Chemical Leaman Tank Lines, Inc. (Chemical Leaman) (defendant), was driven by John Johnson, who survived as the sole eyewitness to the accident. Johnson claimed that the accident was caused by Conway crossing the center line onto Johnson’s side of the highway. Conway’s widow, children, and Dixie Transport (plaintiffs) brought a diversity suit in tort against Chemical Leaman. At a jury trial, expert witnesses for each side offered conflicting evidence as to which truck crossed the center line. The jury rendered a verdict for plaintiffs, but it was overturned on appeal, on the basis of the court’s failure to admit certain evidence. Near the close of the parties’ second jury trial, after plaintiffs had presented their case, Chemical Leaman called an expert witness, Arnold Hay, who had not testified in the first trial or been designated as a witness in the second. Plaintiffs objected to Hay’s testimony. The court sustained plaintiffs’ objection while allowing the testimony to be offered. Hay presented an entirely new theory of the accident in which both trucks came so close to the center line that their mirrors hit, causing Conway to lose control. In its answers to interrogatories, the jury found both Conway and Johnson to be a proximate cause of the accident, a finding that clearly derived from Hay’s testimony. A verdict was rendered for Chemical Leaman. Plaintiffs appealed on the grounds that the interrogatories did not support one party over the other and that the court should not have allowed Hay to testify. The court granted a new trial on the basis of the interrogatories and did not address the argument regarding Hay. The jury at a third trial rendered a verdict for plaintiffs. That verdict was reversed on the ground that the court should not have granted the third trial because the interrogatories at the second trial supported judgment for Chemical Leaman. Plaintiffs then moved for a new trial based on the admission of Hay’s testimony in the second trial. The trial court granted plaintiffs’ motion and then entered judgment in their favor on the basis of the verdict reached in the third trial. Chemical Leaman appealed the grant of a new trial.